core: add artwork theming

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
core: load pixmaps and buttondirs

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
core: cleanup artwork path handling

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
core: add pixmaps and buttons

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>